Revealing mistake: In "Wonderful, Wonderful Day" there are live birds flying on the set, but they were confused by the backdrop and several crash into it. (The director acknowledges this in a documentary.)
Revealing mistake: In the scene where Milly is singing Wonderful Day and the bird come flying past her, their shadow falls on the "hills" behind them.
Revealing mistake: During winter at the cabin, several of the kidnapped girls are doing chores. As they are about to go back inside the cabin, the long icicles hanging on the porch eaves can be seen swinging loosely when they should be frozen in place.
Answer: Yes it is true. They did this to prevent the birds from flying into the backdrops and becoming injured or killed.